1. Field of the Invention
The present invention relates to a friction stir welding technique for longitudinally welding butted metal members with complicated (i.e., intricately) shape portions in section at end portions, and more specifically, relates to a friction stir welding method and metal material welded body manufactured by the method.
2. Description of the Related Art
Recently, a friction stir welding (FSW) technique has been developed and is in practical use as a new technique for joining members to be welded made of metal in place of welding and brazing technique.
The friction stir welding is a solid-phase joining technique in which members to be welded are stirred, softened, and plasticized without melting. In the friction stir welding, metal members as the members to be welded can be joined together with less thermal strain and less strength reduction in a base material than those in welding (fusion joining) and brazing (liquid phase-solid phase reaction joining) methods. Thus, weld failures caused by thermal deformation in welding or oxidation of a welded portion are advantageously reduced.
The friction stir welding is a joining technique in which a columnar or cylindrical joining tool called an FSW tool (referred hereinafter to as a welding tool) is pressed against the members to be welded while being rotated at high speed, and the members to be welded are stirred, softened, and plasticized to be joined together by use of frictional heat generated between the welding tool and the members to be welded.
In the normal friction stir welding, a weldable shape is limited to a flat surface or a gently curved surface due to its welding principle and the shape of the welding tool as disclosed in Patent Document 1 (Japanese Patent Laid-Open Publication No. 2011-101891) and Patent Document 2 (Japanese Patent Laid-Open Publication No. 2011-101890). If the members to be welded are provided with complicated shape portions such as an angular shape, a small round shape, and a stepped shape, the members to be welded are difficult to butt and longitudinally weld together by the normal friction stir welding technology.
Further, as shown in FIG. 1 (1A, 1B) and FIG. 2, in the friction stir welding performed to members to be welded with complicated shape portions such as an outer tube portion, weld surfaces (butt surfaces) 2 of the members to be welded 1 (1a and 1b) are welded through a plurality of welding steps which are divided in accordance with a complicated shape portion (an angular portion) 3 of the members.
However, in the friction stir welding technique, a friction stir welded portion 4 obtained by stirring, melting, softening, and fusing the members to be welded 1 (1a and 1b) is not formed over the entire butt surfaces 2 of the members to be welded 1, and an unwelded portion (portion which is not welded) 5 remains in the complicated shape portion 3. In the members to be welded 1 (1a and 1b), a probe hole 7 of a welding tool 6 also remains in a weld-end edge portion, or the unwelded portion 5 is formed and remains in the complicated shape portion (the angular portion) 3 after the respective welding steps as shown in FIG. 3. In such welding process, although a metal material welded body 8 having an angular shape is provided by butt-welding the members 1 (1a and 1b), the probe hole or the unwelded portion remains in a member surface of a stress concentration portion at which stress is concentrated in the welded body 8.
Patent Document 3 (Japanese Patent Laid-Open Publication No. 2006-239720) also discloses a welding method using a double-acting type friction stir welding apparatus 9 as shown in FIGS. 4A and 4B as a method for eliminating the probe hole 7 in the metal material welded body 8 for the friction stir welding technique. This welding method is performed by the following manner. The double-acting type friction stir welding apparatus 9 uses an elevating device that respectively separately lowers and raises a shoulder portion 9a and a probe portion 9b. The probe portion 9b is pulled up from the probe hole 7, and the shoulder portion 9a is simultaneously pushed down into the members to be welded 1 (1a and 1b) as workpieces after the welding process. A workpiece material is thereby caused to plastically flow into the probe hole 7 so as to fill the probe hole 7.
Furthermore, Patent Document 4 (Japanese Patent Laid-Open Publication No. 2011-121090) discloses a friction stir welding technique for a joint structure in which hollow members to be welded made of metal are butt-welded together by preventing deformation due to a tool load or the like as a welding example between the metal members with the complicated shape portions.
In the friction stir welding technique disclosed in Patent Document 3, however, when the members to be welded 1a and 1b have a large weld depth, the shoulder portion 9a needs to be pushed down more deeply so as to fill the probe hole 7. As a result, a shoulder hole 7a with a larger diameter remains even though the probe hole 7 is made shallow. The probe hole 7 may cause a greater strength reduction. Further, in the friction stir welding technique in Patent Document 4, it is necessary to form a protrusion on one of the members to be welded, and separately provide a support that supports the protrusion by receiving the protrusion on another one of the members to be welded so as to maintain formability of the metal members to be welded.
As described hereinbefore, in the conventional friction stir welding technique, the probe hole or the shoulder hole with a larger diameter remains in the weld-end edge portion, and the unwelded portion is formed and remains in the complicated shape portion (the angular portion) at a time when the members to be welded are butted and longitudinally welded together by using the welding tool, thereby providing poor weld strength, achieving insufficient mechanical and physical strength, and deteriorating appearance quality, thus being disadvantageous.
It is also necessary to separately additionally form the protrusion to one of the members to be welded, and the support for a protrusion, to which the protrusion is fitted, to the other one of the members to be welded in advance so as to suppress deformation and maintain the formability at a time when the metal members to be welded are butt-welded together. Thus, by the conventional method, it is difficult and time-consuming to machine butt-weld portions (joints) of the members to be welded since the butt-weld portions have an intricate (complicated) shape.